The current flowing through the inductor of a switched DC/DC converter displays a ripple at the switching frequency. With regard to its magnetic properties, the inductor is designed such that amperages of the current flowing in normal operation of the DC/DC converter do not saturate its core magnetically. This design aspect determines the minimum size and thus the cost of the inductor. Generally, the operation range of amperages not magnetically saturating the inductor is symmetric with regard to a current of zero ampere and thus independent of the flow direction of the current. The current flowing through the inductor of a DC/DC converter, however, only has one direction. As a result only one half of the usable operation range of its inductor is used. Inductors of DC/DC converters are also referred to as inductors for DC applications or DC inductors here.
It is known to shift the operation range of an inductor apparatus by means of placing a permanent magnet into its magnetic circuit that is defined by its core. Particularly, the magnetic field of the permanent magnet is oriented in an opposite direction to the magnetization which is generated by the direct current flowing through the inductor winding. This measure is referred to as pre- or bias-magnetization or as (magnetically) biasing the inductor. By means of this measure, the magnetic field generated by the direct current is at least partially compensated, and the full operation range of the inductor can be used. This means that the inductor may be made considerably smaller and of considerably less material at an unchanged high efficiency. Thus a cost advantage is achieved as compared to inductors without bias magnetization.
However, there is a considerable risk that even a high-quality permanent magnet loses its magnetization if it is subjected to high temperatures and/or if the field strength of a magnetic field generated by the inductor winding and having a direction opposite to the magnetization of the permanent magnet becomes too high, i.e. higher than the so-called intrinsic coercive field strength of the permanent magnet at the respective temperature. As a result, the level of pre-magnetization may be changed in a disadvantageous way locally or even over the entire inductor apparatus. Such high magnetic field strengths usually do not occur during normal operation of an inductor apparatus, but they may occur under extreme operating conditions. Further, the behavior of the magnetization of a permanent magnet subjected to a magnetic field generated by a current through the inductor winding modulated at a high frequency, particularly in an inductor of a boost converter, is not predictable, and it could have a negative influence on the magnetization of the permanent magnet even if an absolute value of the field strength of such a high-frequency magnetic field is acceptable.
A boost converter comprising an inductor apparatus which includes a permanent magnet in its magnetic circuit is known from EP 0 735 657 B1. A core of the inductor apparatus is magnetically biased by means of a permanent magnet generating a bias magnetization in an direction opposite to the magnetization which is generated by a pulsed direct current flowing through the inductor winding in operation of the boost converter. This allows for use of a comparatively small inductor apparatus as compared to the maximum amperage of the pulsed direct current.
A further inductor apparatus comprising a permanent magnet in its magnetic circuit is known from EP 1 321 950 A1. This document relates to the material requirements which the permanent magnet should fulfill in order to yield both a reduction in volume and an increase in efficiency by implementing a pre-magnetization of the core.
From EP 2 012 327 A2 an inductor apparatus comprising a permanent magnet in its magnetic circuit is known in which the magnetic flux through its core is increased by orienting the permanent magnet at a slant angle. The purpose of this arrangement is to enable the use of plastic-bonded, easily machinable magnet materials for pre-magnetising the core, although they do not comply with certain magnetic requirements. Further, it is exploited that due to their low electrical conductivity no eddy currents are generated in these materials even if subjected to a magnetic field oriented at a right angle to the permanent magnet.
U.S. Pat. No. 6,639,499 B2 describes how to select a geometric arrangement which avoids de-magnetization of the permanent magnet in a magnetic circuit of an inductor apparatus under all conceivable operation conditions of the inductor apparatus. This selection shall allow for using permanent magnets of materials of comparatively low intrinsic coercive field strength. However, no conventional core shapes can be used here, as the center limb of the core has to be longer than the outer limbs.
AT 215 023 B discloses an apparatus for adjusting the inductance of at least one inductor winding arranged on a core made of a magnetically soft, ferromagnetic material. The magnetically soft core is magnetically coupled to at least one further core made of a permanently magnetic material. The magnetic coupling results in a pre-magnetization of the magnetically soft core which in turn has an influence on the inductance of the inductor winding. This influence is adjustable by means of a magnetization winding arranged on the permanently magnetic core. This magnetization winding may be subjected to magnetising or de-magnetising pulses affecting the magnetization of the permanently magnetic core and thus the pre-magnetization of the magnetically soft core. Due to the coupling of the permanently magnetic core to the magnetically soft core, a pre-magnetization of the magnetically soft core results which always reduces the threshold amperage of the current flowing through the inductor winding, i.e. the amperage at which the magnetically soft core is magnetically saturated, independently of the direction of the current through the inductor winding and independently of the direction or orientation of the magnetization of the permanently magnetic core. The apparatus known from AT 215 023 B is used to tune the resonance inductance of a resonance circuit of a receiver for radio or television signals. An inductor used in such a resonance circuit is not subjected to a power current as high as such currents usually occurring in a DC/DC converter or in an EMC filter.
There still is a need for an inductor apparatus suitable for a power current in which a bias magnetization of its core may be used to a maximum extent under various operation conditions to reduce the size of the inductor and thus its cost of production.